Variable speed sheaves have been made for years with boundary or dry lubricating bearings of various materials, such as bronze, graphite, TFE-filled materials, nylon, acetal, wood and many others. Such bearing materials have been utilized for variable speed sheaves in both open belted and enclosed belted assemblies. However, the numerous prior art proposals have continued to fall short of meeting the several requirements necessary to achieve satisfactory sheave life. These requirements are, briefly, as follows:
(1) The radial clearance between the bore of the dry bearing in the flange and the outside diameter of the sleeve must be minimal to limit angular displacement or cocking of the flange due to belt tension. Angular displacement of the flange is directly proportional to radial clearance, as fixed by the geometry of a given sheave. If the initial radial clearance is too large, two serious problems result. First, the angular displacement of the flange becomes great enough to destroy the V-belt and, second, the life of the dry lube bearing is drastically shortened. It is well known that the radial clearances of any dry lube bearing increase as a parabolic (squared) function with respect to time. As the radial clearances become greater, the flange vibrates and generates noise. Dry lube variable speed sheaves now on the market employing conventional dry bearing materials, as mentioned above, require relatively large initial clearances to compensate for inside diameter "close-in" resulting from heat. The final radial clearances, however, are still too great (.002 inch or more) to promote good bearing life of 10,000 hours or more.
The present invention allows the making of a dry lube variable speed sheave which insures as close a radial fit as required by the particular material. In some instances, a very slight interference fit in the sheave assembly is desirable to compensate for bearing material creep or deformation. The invention also assures repeatability in the attainment of the dry bearing bore size or inside diameter as required by the sheave geometry. Most importantly, the method assures an accurate bore for the dry bearing regardless of the thickness tolerance of any bearing component. Conventional designs are much more dependent on the manufacturing tolerances of the bearing assembly components.
(2) Another requirement for a dry lube variable speed sheave is the axial retention of the bearing material in the adjustable flange bore. Some conventional designs employ a pressfit of graphite, bronze or the like into the flange bore. The disadvantage of this lies in the cost of obtaining close tolerance diameters to achieve the required interferences and the fracture possibility for the bearing component during insertion. The present invention completely alleviates this difficulty in the prior art by use of a dry lube bearing component of proper length which after preparation (etching, etc.) is chemically bonded with an epoxy cement in the adjustable flange bore.
(3) A third and critical requirement for a long life sheave of the contemplated type relates to the compressive stiffness of the dry bearing material. The bearing material must exhibit high compressive stiffness and low permanent deformation (creep) under load. The invention permits the use of a thin dry bearing material of high compressive stiffness and low permanent deformation to reduce angular displacement or cocking of the adjustable flange.
(4) The invention eliminates the necessity for machining the bore of the adjustable flange to receive a dry bearing. Machining bores to close tolerances is expensive and is an important competitive factor in connection with variable speed sheaves. There is no known unit currently on the market which does not require machining of the flange bore or bores and thus the invention meets this usual requirement by eliminating it.
Other novel features and advantages of the invention will become apparent during the course of the following detailed description.